March 28, 2013

Older Stars Help Baby Stars Grow Up To Be Massive

Stars with ten times or more the mass of our Sun should not exist. They push away the gas they feed on as they grow, starving themselves for fuel. Astrophysicists have been struggling to understand how some stars are able to overcome this developmental hurdle.

A group of researchers led by the University of Toronto suggests that baby stars might grow to have great mass if they are lucky enough to be born within a corral of older stars. With a favorable arrangement of the older stars, the baby stars are confined and fed gas by the older stars surrounding them. Astronomers have seen evidence of this collective feeding, or convergent constructive feedback, in a giant cloud of dust and gas called Westerhout 3 (W3). W3 is located 6,500 light years from Earth.

Rivera-Ingraham and her colleagues used high-quality and high-resolution far-infrared images from the European Space Agency's (ESA) Herschel Space Observatory to study the formation of high-mass stars. Herschel's two cameras recorded light invisible to the naked eye, spanning a range from infrared radiation partway into the microwave region.

A group of scientists, including Peter Martin, Professor in the Canadian Institute for Theoretical Astrophysics at the University of Toronto, exploited the cameras' abilities to create the HOBYS Key Program. HOBYS is used to study the birth of very massive stars in nearby giant clouds of gas and dust inside our own galaxy, including W3.

By mapping the density of dust and its temperature, looking for the most dense regions where the dust is shielded and cold, scientists are able to track the regions of the gas cloud where stars are about to form.

“We can now see where stars are about to be born before it even happens, because we can detect the cold dust condensations,” says Martin. “Until Herschel, we could only dream of doing that.”

In the denser parts of the gas clouds where gas is compressed enough to trigger nuclear fusion, stars are born. The more massive these new stars are, the more visible and ultraviolet light is emitted, heating up the surroundings of the baby star — including the dust studied by Herschel.

“The radiation during the birth of high-mass stars is so intense that it tends to destroy and push away the material from which they need to feed for further growth,” says Rivera-Ingraham. This process has been modeled by scientists who found that stars approximately eight times the size of our Sun stop growing because they run out of gas.

The problem is that astronomers see stars that are more massive than this theoretical limit allows. The team has found clues to how this might be possible by studying W3.

They noticed that a congregation of old high-mass stars surrounded the densest region of the cloud. The scientific team suggests that this placement is exactly how those young high-mass stars survive and grow. Each generation of stars might have created the right conditions for another generation to grow comparably or even more massive in its midst, ultimately leading to the formation of a rare, massive cluster of high-mass stars.

Older stars radiate and push gas away just like the baby stars do. If a group of such older stars happens to be arranged favorably around a major reservoir of gas, they can compress it enough to ignite new stars. New, high-mass stars are born because of this corralling of dense gas.

A large, freshly ignited star will push its food source away. If surrounded by enough large older stars, they will keep nudging gas back at the baby, feeding it. With such collective feeding, it is easy to see how such a baby could become massive.

The team hopes to continue its research by simulating the situation with computer modeling, by measuring gas motions, and by comparing their results with data from other giant clouds studied by HOBYS. This will allow them to discern the mechanism that gives rise to high-mass stars in these giant clouds, whether or not it is collective feeding.